The US says it will respond "swiftly and appropriately" if Syrian President Bashar Assad uses chemical weapons "again," suggesting that Damascus was behind an April "attack" which Moscow says was entirely staged.

"The United States is closely monitoring the situation in Idlib province, Syria, where millions of innocent civilians are under threat of an imminent Assad regime attack, backed by Russia and Iran," the White House statement reads.

"Let us be clear, it remains our firm stance that if President Bashar al-Assad chooses to again use chemical weapons, the United States and its Allies will respond swiftly and appropriately."

The statement comes just two days after Russia said that Syrian rebels - not the government - were planning to stage a chemical attack in Idlib province. Even so, Russian Defense Ministry Spokesman Igor Konashenkov said that such an attack will be blamed on Damascus and will "serve as another reason for the US, the UK and France to hit Syrian government targets with air strikes.”

According to Konashenkov, the jihadist group Hayat Tahrir al-Sham is “preparing another provocation of the ‘use of chemical weapons’ by Syrian government forces against the peaceful population of the Idlib province.” He said the group has delivered "eight chlorine tanks" to Jisr al-Shughur to "stage" the attack, and that the tanks were later taken to a village eight kilometers (5 miles) away.

He also said that British services are "actively involved" in the upcoming "provocation," adding that a group of militants "trained in handling poisonous substances under the supervision of specialists from the private British military company Oliva" arrived in Jisr al-Shughur.